I do not own a Fitbit, but on my older Pebble, the music integration feature allows me to play/pause the music, switch tracks (to skip that one song in the playlist I should really remember to remove), and adjust the volume.

When I'm working out or doing yardwork, it's handy to have access to these things on my wristwatch rather than trying to fiddle with the phone in my pocket. It's sounds like the most minor convenience, but I use it all the time. If that were to be limited to only a tiny number of apps (and not the Android bluetooth thing, which picks up whatever happens to be playing; usually VLC in my case, sometimes Spotify) it would be a lot less useful.